51. ORGANIC GEOCHEMICAL COMPARISON OF CRETACEOUS BLACK SHALES AND ADJACENT STRATA FROM DEEP SEA DRILLING PROJECT SITE 603, OUTER HATTERAS RISE 1

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1 51. ORGANIC GEOCHEMICAL COMPARISON OF CRETACEOUS BLACK SHALES AND ADJACENT STRATA FROM DEEP SEA DRILLING PROJECT SITE 603, OUTER HATTERAS RISE 1 Keith W. Dunham, Philip A. Meyers, and Pamela L. Dunham, The University of Michigan 2 ABSTRACT Organic matter contents of black shales from the Cretaceous Hatteras and BlakeBahama formations have been compared to those from surrounding organicpoor strata using C/N ratios, δ 13 C values, and distributions of extractable and nonsolventextractable, longchain hydrocarbons, acids, and alcohols. The proportion of marine and landderived organic matter varies considerably among all samples, although terrigenous components generally dominate. Most black shales are hydrocarbonpoor relative to their organiccarbon concentrations. Deposition of the black shales in Hole 603B evidently occurred through turbiditic relocation from shallower landward sites and rapid reburial at this outer continental rise location under generally oxygenated bottomwater conditions. INTRODUCTION Occurrences of darkcolored layers of Cretaceous rocks having relatively high concentrations of organic matter have been found in numerous locations studied as part of the Deep Sea Drilling Project (DSDP). The distribution of such occurrences in the North Atlantic Ocean has been discussed by Arthur (1979), Tucholke and Vogt (1979), Thierstein (1979), Graciansky et al. (1981), Weissert (1981), and Waples (1983), among others, with the intent of identifying the paleoceanographic factors involved in the formation of these unusual strata, commonly called "black shales." Improved preservation of organic matter, increased contribution of continental organic matter to oceanic basins, and enhanced production of marine organic matter are some of the factors that have been suggested. Because these three possibilities affect the character of the organic content of black shales, the organic matter in North Atlantic examples has been investigated; these investigations are summarized by Tissot et al. (1980), Summerhayes (1981), Katz and Pheifer (1982), and Graciansky et al. (1982). Varying proportions of marine and terrigenous organic constituents are found in sediments deposited at different times and locations in the Cretaceous Atlantic Ocean. A comparison of the organic matter contained within black shales with that of the adjacent organiccarbonpoor lithologies further contributes to this information. In this chapter, we describe comparisons of analyses done on black shales and closely bedded strata from the Hatteras and BlakeBahama formations in the western North Atlantic Ocean. Methods Twentyfive Hole 603 B samples from different ages in the Cretaceous were selected on board Glomar Challenger for this study. These van Hinte, J. E., Wise, S. W., Jr., et al., Init. Repts. DSDP, 93: Washington (U.S. Govt. Printing Office). 2 Address: Oceanography Program, The University of Michigan, Ann Arbor, MI were augmented with postcruise sampling from frozen core sections. Five sample groups contain closely bedded organiccarbonrich and organiccarbonlean strata. Hatteras Formation samples consist of one Turonian sample, six Cenomanian samples, and eight samples. Three samples of Barremian age, five Hauterivian, and one Valanginian sample are from the BlakeBahama Formation. All samples were frozen immediately after collection and remained frozen until analysis began. The samples were freezedried for determination of their total carbon contents with a HewlettPackard 185B CHN Analyzer. Residual carbon was measured after HC1 dissolution of carbonates and was considered to represent the total organiccarbon content. Percent calcium carbonate was calculated from the difference between initial and residual carbon contents. Organicmatter atomic C/N ratios were determined from residual carbon values. Organiccarbon contents of the samples were calculated on a dryweight basis (%) for the original, carbonatecontaining sediment. Stable carbon isotope ratios of the organiccarbon content of these samples were determined on carbonatefree samples using a VG Micromass 602 mass spectrometer calibrated with NBS20 (carbonate) and NBS22 (petroleum) standards. Data are corrected for 17 O and are presented relative to the PDB standard. A twostage extraction procedure was used to obtain the geolipid contents. Soxhlet extraction with toluenemethanol yielded the easily extractable, or free, lipids. A second extraction with 0.5N KOH in methanoltoluene provided the hydrolyzable, or bound, geolipids. Both fractions were treated with methanolic boron trifluoride to convert fatty acids to their methyl esters. Geolipid subfractions were separated by column chromatography on alumina over silica gel. The subfractions obtained contained alkanes and alkenes, aromatic hydrocarbons, fatty acid methyl esters, and hydroxy lipids including sterols and alkanols. Hydroxy compounds were silylated with bistrimethylsilyltrifluoroacetamide (BSTFA) prior to gas chromatography. Splitless injection gasliquid chromatography was employed to determine the types and amounts of components present in the geolipid subfractions. A HewlettPackard 5830 FID gas chromatograph equipped with a 20m SE54 fused silica capillary column was used with hydrogen as the carrier gas. Quantification was achieved through the use of known amounts of internal standards added to each sample before column chromatography. Individual compounds are tentatively identified by retention times in this preliminary survey. Reported values have been corrected for mass discrimination over the wide molecularweight range reported. LITHOLOGIC SETTING Cretaceous strata in the North Atlantic have been divided into the Plantagenet Formation, the Hatteras Formation, and the BlakeBahama Formation on the basis 1195

2 K. W. DUNHAM, P. A. MEYERS, P. L. DUNHAM of earlier DSDP sampling (cf. Jansa et al., 1978; Sheridan et al., 1983; Summerhayes and Masran, 1983). The rocks comprising these formations at Site 603 are detailed in the Site 603 chapter (this volume) and are only briefly described here. s were found in rocks ranging in age from Santonian to Berriasian at Hole 603B on the outer Hatteras Rise in the North American Basin (Fig. 1). Neither the lithologic settings nor the organiccarbon contents of these deposits are uniform over this considerable time span, although the common presence of turbidites and of bioturbation show some important similarities existed in the depositional environments. Santonian to late Turonian rocks consist of variegated claystones with sparse black shales and correspond to the Plantagenet Formation. The early Turonian to Aptian section contains abundant black shales interspersed among red and green claystones and corresponds to the Hatteras Formation. s and sandstones with black claystone turbidites make up the AptianBerriasian Blake Bahama Formation. Cenomanian black shales contain the highest concentrations of organic carbon (Herbin et al., this volume; Meyers, this volume), and all black shales generally exist as thin strata surrounded by organiccarbonpoor rocks. RESULTS Organic Carbon, C/N Values, and Carbon Isotopes Hatteras Formation The seven black shales of the Hatteras Formation have a significantly higher concentration of organic carbon than the adjacent lightercolored strata. Organic carbon averages 2.71% in the black shales and 0.28% in the adjacent strata (Table 1). Mclver (1975) compiled an average value of 0.3% organic carbon for ancient deepocean sediments from DSDP Legs 1 through 33. This value may be considered the background level for normal deepocean sediments. The high concentrations found in the black shales, therefore, indicate unusual circumstances for their deposition. C/N ratios of Hatteras Formation black shales have averages significantly higher than adjacent green and red claystones. Based upon a survey of marine sediments, 45 N r i i i i i i I i i DSDP Sites Legs 2, 11, 43,44, 76, 93 40' 35' Location of DSDP Site 603 in relation to other DSDP sites in the western North Atlantic Ocean. 1196

3 ORGANIC GEOCHEMICAL COMPARISON OF CRETACEOUS BLACK SHALES Table 1. General descriptions of Cretaceous samples selected for organic geochemical comparison, Hole 603B, Hatteras and BlakeBahama formations. CoreSection (interval in cm) Subbottom depth (m) Age Lithology CaCO 3 (%) org C/N δ C or g Hatteras Formation 33.CC, , , , , , , , , , , , , , , , Turonian Cenomanian Cenomanian Cenomanian Cenomanian Cenomanian Cenomanian Redgreen claystone Red claystone Red claystone BlakeBahama Formation 492, , , , , , , , , Barremian Barremian Barremian Hauterivian Hauterivian Hauterivian Hauterivian Hauterivian Valanginian Sandstone > Premuzic et al. (1982) suggest that C/N ratios less than 8 indicate mostly marine organic matter, and values greater than 15 show a predominance of landderived material. With increasing time of burial, however, C/N ratios change as a result of diagenesis. Waples and Sloan (1980) report a gradual decrease in C/N values from about 10 in Quaternary sediments to about 4 in Miocene samples, followed by increases in older sediments. Furthermore, C/N values tend to be high in sediments where marine organic matter is well preserved, such as in Cenomanian black shales from the Angola Basin (Meyers et al., 1984). Carbon isotope data provide further information about the sources of organic matter in these samples. In general, landderived organic matter is more depleted in 13 C than is marine organic matter, although carbon isotope ratios appear to be sensitive to diagenetic modification in black shale deposits and hence should not, by themselves, be considered absolute determinants of source (Dean, Claypool, et al., 1984; Meyers et al., 1984). Hatteras Formation black shales average 25.9%o and range from 23.8 to 27.7%o. Adjacent green and red claystones have a wide range of δ 13 C values, from 22.0 to 29.9%o. BlakeBahama Formation Organiccarbon percentages of the five BlakeBahama Formation black shales average 1.92% (Table 1). s and sandstones adjacent to the black shales average slightly lower values, with the exception of limestone Sample 603B662, cm, which has a value of 0.04% organic carbon. C/N ratios in the BlakeBahama Formation are relatively high, averaging There are two large deviations from this average. Sample 603B715, cm has an undetectable amount of nitrogen; therefore, the C/N ratio is reported as greater than. Sample 603B 662, 6870 cm has a C/N value of 6.3 (Table 1). BlakeBahama black shales have δ 13 C values averaging 25.O%o. Adjacent lithologies once again show a wide range of values from 23.6 to 28.7%o (Table 1). Extractable Alkanes, Fatty Acids, and Alkanols Hatteras Formation Concentrations of extractable (free) and bound geolipids are given in Tables 2 and 3 in terms of parts per million of dry sample weight and also relative to the organiccarbon concentrations. In comparison to samples from DSDP Site 530 in the Angola Basin, the organiccarbonlean samples contain about the same amounts of n alkanes, but are richer in alkanoic acids and alkanols (Meyers et al., 1984). The black shale samples, in contrast, contain substantially lower concentrations than the black shales from the Angola Basin (Meyers et al., 1984) and often less than adjacent strata. It is evident that these black shales of the Hatteras Formation are geolipidlean. The free and bound geolipid distribution of the black shales and adjacent strata are compared in Figures 2 through 9. Longchainlength geolipids, such as C 2 7C 33 oddnumbered rtalkanes and C24C32 even numbered n alkanoic acids and nalkanols, are present in all samples 1197

4 K. W. DUNHAM, P. A. MEYERS, P. L. DUNHAM Table 2. Concentrations of solventextractable (free) and nonsolventextractable (bound) geolipid fractions obtained from Hole 603B samples from the Hatteras and BlakeBahama formations. CoreSection (interval in cm) Hatteras Formation 33.CC, , , , , , , , , , , , , , 4245 BlakeBahama Formation 492, , , , , , , , , Lithology Redgreen claystone Red claystone Red claystone Sandstone Free nalkanes Bound _ πalkanoic acids Free Bound πalkanols Free _ Bound Note: All concentrations given in micrograms of geolipid components, as measured by gas chromatography, per gram dry weight of sample. Dash indicates that the concentration was not measured and are characteristic of land plant waxes (Simoneit, 1978). Shortchainlength walkanes, such as C 17 C 2 i, are found more often in the bound fraction. These shortchainlength /7alkanes can be indicative of algal input (Simoneit, 1978). Comparison of the seven black shale samples reveals one dominant geolipid feature. A large proportion of terrigenous components is present in both the free and bound fractions. The importance of landderived geoliopids is especially conspicuous in the nalkane distributions, which in this regard resemble distributions reported for black shales sampled from the Hatteras and BlakeBahama formations at DSDP Sites 391 and 534, farther south of Site 603 in the North American Basin (Deroo et al., 1978; Erdman and Schorno, 1978; Stuermer and Simoneit, 1978; Herbin et al., 1983). The ratio of the isoprenoid hydrocarbons, pristane and phytane, is generally less than one in all of the Hatteras Formation samples. Both the free and bound fatty acid distributions are dominated by «C 16, which is a ubiquitous component characteristic of all biota (Figs. 2 through 9). Distribution of «fatty acids between black shales and adjacent strata are very similar. The black shales show a strong landderived geolipid signature. These distributions are similar to a black shale at Site 391, also from the Hatteras Formation (Cardoso et al., 1978). Alkanol distributions in the Hatteras Formation show little variation between black shales and adjacent green and red claystones. Common to both the free and bound fractions in all of the samples is the dominance of the Table 3. Geolipid concentrations from Hatteras and BlakeBahama formation Hole 603B samples relative to organic carbon concentrations. CoreSection (interval in cm) Lithology Free MAlkanes Bound nalkanoic acids Free Bound nalkanols Free Bound Hatteras Formation 33,CC, , , , , , , , , , , , , , 4245 Redgreen claystone Red claystone Red claystone _ BlakeBahama Formation 492, , , , , , , , , Sandstone , Note: Solventextractable (free) and nonsolventextractable (bound) concentrations in parts per million are divided by percent organic carbon for each geolipid fraction. Dash indicates that the concentration was not measured. 1198

5 ORGANIC GEOCHEMICAL COMPARISON OF CRETACEOUS BLACK SHALES 603B33, CC, 118 cm Free fraction l! I,ll I III, l.i l.l I 603B342, 8587 cm nil I ill I I II I ill ill I. i. i i i i 603B , eni 1 I1 III ill. 1 I ill πalkaπe chain length nfatty acid chain length πalkanol chain length Figure 2. Distributions of free (solventextractable) geolipids from three Hatteras Formation black shales. Relative abundances are normalized to the major component in each sample. Isoprenoid hydrocarbons pristane and phytane are presented as dotted and dashed lines, respectively. 603B33, CC, 118 cm Bound fraction II II 1,1,1.1 I i 603B342, cm III B423, cm L L i i. l i l i l. J i. i. l. l i nalkane chain length nfatty acid chain length nalkanol chain length Figure 3. Distributions of bound (nonsolventextractable) geolipids from three Hatteras Formation black shales. See Figure 2 legend for details. 1199

6 K. W. DUNHAM, P. A. MEYERS, P. L. DUNHAM cm Section 603B374, free traction I 1 I II cm.him11 II I.ll I I I.. 1 l i, cm < l l l l l 1 1 III πalkane chain length nfatty acid chain length πalkanol chain length Figure 4. Distributions of free (solventextractable) geolipids from three closely spaced Cenomanian samples from the Hatteras Formation. See Figure 2 legend for details cm Section 603B374, bound fraction I I I111...I.I.. ll I ill cm > I III 1 1 illl l. I III ll II I I cm " I I III ll ll ll πalkane chain length nfatty acid chain length πalkanol chain length Figure 5. Distributions of bound (nonsolventextractable) geolipids from three closely spaced Cenomanian samples from the Hatteras Formation. See Figure 2 legend for details. 1200

7 ORGANIC GEOCHEMICAL COMPARISON OF CRETACEOUS BLACK SHALES " cm Redgreen Section 603B384, free fraction i i i i ll α> 8386 cm ll III, Mill i 1,h 1...I.I cm ill! ll III ll 1.ll 1III 1 i l πalkane chain length nfatty acid chain length πalkanol chain length Figure 6. Distributions of free (solventextractable) geolipids from three closely spaced samples from the Hatteras Formation. See Figure 2 legend for details. 56e Redç 0 3 re cm en Section 603B384, bound fraction II, I ll. i i 1 1 1, 8386 cm II lull cm.nil h ii,h I III I nalkane chain length πfatty acid chain length nalkanol chain length Figure 7. Distributions of bound (honsolventextractable) geolipids from three closely spaced samples from the Hatteras Formation. See Figure 2 legend for details. 1201

8 K. W. DUNHAM, P. A. MEYERS, P. L. DUNHAM cm Red Section 603B402,1ree fraction n,1 II.ll ll. I cm ü IllJjJ.I. I I il.i.i cm M J_L. I cm, JJ^J I cm Red I! Illll I I nalkane chain length πfatty acid chain length. 1...I.I 1. πalkanol chain length Figure 8. Distributions of free (solventextractable) geolipids from five closely spaced samples from the Hatteras Formation. See Figure 2 legend for details. C 2 8 and C 30 /talkanols. Longchain /2alkanols such as these have been interpreted to be indicators of terrigenous geolipids in marine sediments (Brassell et al., 1982). The majority of the samples contain large quantities of C 2 2 walkanol, which has been observed in other deepocean organiclean sediments (Keswani et al., 1984) and may result from microbial processing of sediment organic matter. BlakeBahama Formation Concentrations of extractable and bound geolipids are given in Tables 2 and 3 in terms of parts per million of dry sample weight and relative to the organiccarbon concentrations. Both the organiccarbonrich and organiccarbonlean samples follow the same observations in the Hatteras Formation. In comparison to the adjacent strata and other deepocean blackshales, these BlakeBahama Formation shales are geolipidlean. Free and bound geolipid information for the Blake Bahama Formation samples are compared in Figures 10 through 15. Differences between the distribution of n alkanes in organiccarbonrich and organiccarbonlean samples are not straightforward. All of the samples, however, have a strong terrigenous geolipid signature with 1202

9 ORGANIC GEOCHEMICAL COMPARISON OF CRETACEOUS BLACK SHALES 262J)c m Re( i Section 603B402, bound fraction k h I I. I. I l l....ill ll I II " cm h I INI n i i i i.i 1 ü i cm I I.I 1 1 1, cm h III HIM i 11 i LI 1,.l cm Red h I ill I...I.I.1., 1 i nalkane chain length πfatty acid chain length πalkanol chain length Figure 9. Distributions of bound (nonsolventextractable) geolipids from five closely spaced samples from the Hatteras Formation. See Figure 2 legend for details. abundant C 2 η, C 2 9, C 31, and C 33 «alkanes. The ratio of the isoprenoid hydrocarbons, pristane and phytane, is generally less than one when available. Unlike the /2alkanes, very little variability is observed in the «fatty acid distributions between individual black shale units and adjacent strata. As in the Hatteras Formation, C 16 nfatty acid dominates the distribution pattern. Significant amounts of longer chain lengths indicate strong landderived wfatty acids (Simoneit, 1978). The distribution of both free and bound rcalkonols shows a terrigenous geolipid dominance. Shortchainlength algal inputs are not observed in the free fraction; however, some of the bound fractions from the Blake Bahama Formation contain C 16 and Qg «alkanols. The presence of C 22 «alkanol is variable. It ranges from low levels to the most abundant nalkanol in Sample 603B 363, cm (Figs. 10 through 15). Hatteras Formation DISCUSSION Organic matter in the black shales is a mixture of marine and varying proportions of continentally derived organic matter. The high C/N values may record selective 1203

10 K. W. DUNHAM, P. A. MEYERS, P. L. DUNHAM 603B492, cm Free fraction. I I h ll l 603B571, cm Nil,h I I. I. 603B675, cm II Illl ill!. l. I πalkane chain length πfatty acid chain length πalkaπol chain length Figure 10. Distributions of free (solventextractable) geolipids from three BlakeBahama Formation black shales. See Figure 2 legend for details. 603B492, cm Bound fraction II li...ll B571, cm α> III I I Ü I I ill li I Mill I I I..I i i 603B675, 8184 cm II II II II 1 1, πalkane chain length πfatty acid chain length πalkanol chain length Figure 11. Distributions of bound (nonsolventextractable) geolipids from three BlakeBahama Formation black shales. See Figure 2 legend for details. 1204

11 ORGANIC GEOCHEMICAL COMPARISON OF CRETACEOUS BLACK SHALES 6870 cm Section 603B662, free fraction ll Lull i. i.1. i in.. i.i.i i.i. i cm <u I...Ml. I ill _u_l cm al llü J_L J_L Ilil lü Ash πalkane chain length πfatty acid chain length nalkanol chain length Figure 12. Distributions of free (solventextractable) geolipids from three closely spaced Hauterivian samples from the BlakeBahama Formation. See Figure 2 legend for details. 30 n 6870 cm Section 603B662, bound fract on 1π i n _LL. I i 1 ill h cm III! l I cm mm11 I,...i. i i i. I. I i i 1 πalkane chain length πfatty acid chain length πalkanol chain length Figure 13. Distributions of bound (nonsolventextractable) geolipids from three closely spaced Hauterivian samples from the BlakeBahama Formation. See Figure 2 legend for details. 1205

12 K. W. DUNHAM, P. A. MEYERS, P. L. DUNHAM 603B534, cm Free fraction. I I I l l I,., I I il Mil 603B715, cm uu 603B763, cm, I I, I I.... I l l I h I, n nalkane chain length nfatty acid chain length nalkanol chain length Figure 14. Distributions of free (solventextractable) geolipids from two limestones and one sandstone from the BlakeBahama Formation. See Figure 2 legend for details. 603B534, cm Bound fraction " , B715, cm II I I, B763, cm "...III.., πalkane chain length πfatty acid chain length nalkanol chain length Figure 15. Distributions of bound (nonsolventextractable) geolipids from two limestones and one sandstone from the BlakeBahama Formation. See Figure 2 legend for details. 1206

13 ORGANIC GEOCHEMICAL COMPARISON OF CRETACEOUS BLACK SHALES preservation of carbonaceous components relative to marine organic nitrogenous components. Organic matter in the organiccarbonlean green and red claystones of the Hatteras Formation appears to be made up of detrital, highly oxidized materials similar to those found in most deepsea sediments (cf. Degens and Mopper, 1976). Such sediments commonly have low C/N ratios in older strata. Comparison of carbon isotope compositions of black shales with their adjacent strata shows a positive relationship between higher organiccarbon concentrations and a stronger marine isotope character in black shale samples (Fig. 16A). The isotope values of rocks surrounding the black shales display a wide range that suggests varying origins of the organic matter in these layers of turbiditic, organiccarbonlean rock. This relationship implies that the organicmatter concentration of Hatteras Formation black shales is directly related to burial of greater proportions of marine organic matter superimposed upon a background of poorly preserved organic matter from variable sources. C/N and δ 13 C values are both indicators of the source of organic matter. High C/N values from vascular land plants should theoretically relate to light δ 13 C values and lower C/N values from marine organic matter with heavier δ 13 C values. There is not such a relationship between C/N values and δ 13 C values in these Hatteras Formation samples (Table 1). This further indicates the susceptibility of C/N values, in particular, to diagenetic changes. Geolipid distribution and concentrations in black shales and the adjacent strata are difficult to interpret. There appears to be no emergent geolipid pattern in relationship to other organic geochemical parameters. For example, three black shale samples Samples 603B33,CC, 118 cm; 603B342, 8587 cm; and 603B423, 4243 cm have dissimilar nalkane geolipid distributions (Fig. 2). Sample 603B342, 8587 cm has an «alkane geolipid pattern typical of terrigenous organic matter. The δ 13 C value for this sample, however, is the most marine in character compared to the other two black shales. Part of the explanation for this characteristic may arise from enhanced preservation of nonlipid components of sediment organic matter in black shales. Such dilution of geolipid concentrations is postulated for black shales relatively rich in organic carbon (Meyers et al., 1984). In black shales with relatively low amounts of organic carbon, such as Sample 603B422, 4240 cm, it appears that even the lipid components have been microbially reworked and nearly all of the remaining organic matter is detrital inert matter. Fatty acid and alkanol concentrations in the black shales are high compared to other deepocean black shales (Meyers et al., 1984). The higher amounts of the latter two geolipid fractions may reflect larger proportions of waxy landplant material in the Hatteras Formation samples, which is consistent with the geolipid distributions (Figs. 2 through 9). Turbidites are common throughout the Hatteras and BlakeBahama formations; therefore, transport of continental organic matter to this deepsea location is feasible. It is surprising, however, that the «alkanol distributions are so similar despite considerable differences in total organiccarbon concentrations and in «alkane distributions. Alkanols are considered to be more subject to microbial reworking than are alkanes, with the result that the C 2 2 flalkanol suspected to indicate microbial activity is the major alkanol found in openocean sediments in which organic carbon concentrations are low (Keswani et al., 1984). It seems paradoxical that many of the Hatteras Formation black shales (Figs. 2 through 10) display major contributions 7 A 1 i i i i s shales Red claystones i i B I I i i i I i # s s Sandstone _ 6 1 to o Organic i 5 A 3 _ 2 1 *. f i > f 1 1 i i I I δ 1 3 C Figure 16. Organic carbon percentages vs. carbon isotope values, Hole 6O3B. A. Hatteras Formation samples. B. BlakeBahama Formation samples. δ 13 C 1207

14 K. W. DUNHAM, P. A. MEYERS, P. L. DUNHAM of the C 2 2 /2alkanol, unless these samples originally contained organic matter of the proper type and amount to support microbial activity. Although pristane/phytane values below one have been proposed as an indication of anoxic depositional conditions (Didyk et al., 1978), methanogenic bacteria generate phytane during anaerobic fermentation of organic matter (Risatti et al., 1984). Because such fermentation can occur under anoxic conditions deeper in sediments as well as at an anoxic water/sediment boundary, low pristane/phytane ratios may record bacterial fermentation rather than a specific depositional condition. BlakeBahama Formation Organic matter in the BlakeBahama Formation black shales is predominantly terrigenous with small admixtures of marine organic matter. High C/N values for the BlakeBahama limestones and sandstone suggest these lithologies may contain substantial amounts of coaly material eroded from continental deposits. The RockEval results (Meyers, this volume) show the relatively inert character of organic matter in samples of similar lithologies, even though some are fairly rich in organic carbon. Comparison of carbon isotope compositions for the black shales, limestones, and sandstone does not show the clear relationship with organiccarbon content seen in the Hatteras Formation samples (Fig. 16B). Some relationship, however, may exist in the lightercolored, usually organiccarbonlean samples. The lean samples cluster around This cluster may indicate the normal background load of organic matter highly terrigenous in nature. Sample 603B763, cm is the one exception; however, it does contain a larger amount of organic carbon. All distributions of nalkanes in the BlakeBahama Formation black shales show a strong terrigenous character (Figs. 10 through 15). Shorter chain lengths, when present, do not show a major algal input. The absence of oddchainlength predominance may indicate considerable bacterial reworking (Simoneit, 1978). Fatty acid and alkanol concentrations are high compared to other deepocean blackshales, most likely for the same reasons already outlined for the Hatteras Formation samples. General The proportion of continental and marine organic matter present in black shale samples from the western North Atlantic varies considerably. Although most of the organic matter appears to be terrigenous (Katz and Pheifer, 1982), the marine fraction increases with distance from North America (Tissot et al., 1980; Summerhayes and Masran, 1983). This pattern has been explained by Summerhayes and Masran (1983) to reflect the decrease in turbiditic dilution of marine sediments with continental materials as distance from shore increases. An exception to this generality occurs in Cenomanian black shales, where large proportions of marine organic matter are found at Site 105 (Summerhayes, 1981) located on the continental rise and in turbiditic environments. This exception illustrates the regional and temporal variability that can exists in the mixture of organicmatter types in black shales. On the basis of carbon isotope ratio variability, the organiccarbonlean rocks surrounding black shale samples from Site 603 also appear to demonstrate considerable variation in their proportions of marine and terrigenous organic matter. The paleoceanographic picture that emerges is that during the Cretaceous, the western North Atlantic appears to have been a narrow sea filled with oxygenated water. It appears to have received an abundant supply of terrigenous elastics from North America and to have had at best moderate levels of marine productivity. How did black shales form under such conditions? Whereas it is possible that bottomwater anoxia existed from time to time in the basins comprising the protoatlantic, it is unlikely that enough marine organic matter would survive sinking through the predominantly oxygenated water column to form black shales. It is more likely that the organic material reached the deepocean sites of black shale deposition in the company of turbidity flows, and preservation occurred from rapid burial in the oxygenated deep basins. This scenario of downslope transport and redeposition is essentially the same as earlier proposed by Dean, Arthur, et al. (1984) for Site 530 in the Angola Basin and by Robertson and Bliefnick (1983) for Site 534 in the BlakeBahama Basin, and it helps explain the variability in organic character present in the sediments of the western North Atlantic. This explanation is based upon the assumption that each occurrence of black shale deposition is a local or regional event, loosely linked to other such events by global paleoceanographic conditions. Changes in climate, sea level, or oceanic circulation might destabilize ocean margin sediments and initiate turbidity flows at numerous locations around the North Atlantic. Such flows need not be absolutely synchronous and need not contain the same organicmatter content. Except for those originating where the oxygenminimum layer intercepts the bottom, most would actually be poor in organic carbon; all would have their proportions of marine and terrigenous materials controlled by local conditions. On the seafloor, these flows would form fanlike deposits that would overlap and interfinger, rarely creating the continuous layers like those formed under shallow epicontinental seas. Although each black shale deposit would be initiated by a common set of global or oceanwide conditions, its individual characteristics would be determined by regional or local factors. CONCLUSIONS 1. The content and character of organic matter in Cretaceous black shales from DSDP Site 603 in the western North Atlantic are quite variable. 2. Consistent with their large fraction of terrigenous organic matter, the black shales from Site 603 are relatively poor in extractable alkanes, alkanoic acids, and alkanols. 3. The lack of correspondence in isotope values between the black shales and their adjacent strata, despite 1208

15 ORGANIC GEOCHEMICAL COMPARISON OF CRETACEOUS BLACK SHALES similarities in geolipid distributions, implies differences in sources of the bulk, nonlipid organic matter. 4. The black shales represent short episodes of enhanced burial of organic matter within a general setting of poor preservation of carbonaceous materials. A prevailing oxygenated depositional setting is indicated by faunal burrowing, oxidized minerals, and low concentrations of organic carbon in most of the Cretaceous rocks from the western North Atlantic. 5. A scenario of downslope transport by turbidity flows from ocean margin locations within oxygen minima and redeposition in deepocean settings is invoked to explain black shale formation at Site 603 and other western North Atlantic locations. ACKNOWLEDGMENTS We thank the Deep Sea Drilling Project for providing the samples used in this study and for giving P. A. Meyers the opportunity to participate in Leg 93. We are especially grateful to K. C. Lohman and his staff for the use of the carbon isotope mass spectrometer and to J. Powaser for laboratory assistance. This work was partially supported by the U. S. National Science Foundation. REFERENCES Arthur, M. A., North Atlantic Cretaceous black shales: The record at Site 398 and a brief comparison with other occurrences. In Ryan, W. B. R, Sibuet, J.C. et al., Init. Repts. DSDP, Al, Pt. 2: Washington (U.S. Govt. Printing Office), Brassell, S. C, Eglinton, G., and Maxwell, J. R., Preliminary lipid analyses of two Quaternary sediments from the Middle America Trench, southern Mexico transect, Deep Sea Drilling Project Leg 66. In Watkins, J. S., Moore, J. C, et al., Init. Repts. DSDP, 66: Washington (U.S. Govt. Printing Office), Cardoso, J. N., Wardroper, A. M. K., Watts, C. D., Barnes, P. J., Maxwell, J. R., Eglinton, G., Mound, D. G., and Speers, G. C, Preliminary organic geochemical analyses; Site 391, Leg 44 of the Deep Sea Drilling Project. In Benson, W. E., Sheridan, R. 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16 K. W. DUNHAM, P. A. MEYERS, P. L. DUNHAM Waples, D. W, Reappraisal of anoxia and organic richness, with Weissert, H., The environment of deposition of black shales in emphasis on Cretaceous of North Atlantic. Am. Assoc. Pet. Geol. the Early Cretaceous: An ongoing controversy. In Warme, J. E., Bull., 67: Douglas, R. G., and Winterer, E. L. (Eds.), The Deep Sea Drilling Waples, D. W., and Sloan, J. R., Carbon and nitrogen diagene Project: A Decade of Progress. Soc. Econ. Paleontol. Mineral, sis in deep sea sediments. Geochim. Cosmochim. Acta., 44:1463 Spec. Publ., 32: Date of Initial Receipt: 5 April 1985 Date of Acceptance: 4 December